Plummer is a British safecracker with a long prison sentence for robbery on the Channel Islands. With 14 years yet to serve, he is left in his cell when WW II breaks out and the islands are abandoned after France falls. The Germans come to occupy, and he makes a proposition to his new
captors: he will use his criminal talents to spy for them if they let him out and pay a large fee. The Germans are suspicious but agree, and Plummer is soon sent to France for training by monocled Brynner, who suspects his student's motives. After he passes a loyalty test, Plummer is parachuted
into England on a mission of sabotage. He immediately goes to the British intelligence serv...